In the last decade, fiber-optics-based transmission systems have become ubiquitous, due in part to their large bandwidth and high data rate capability. Wavelength-division-multiplexed (WDM) networks employ a transmission technique that allows multiple wavelengths (i.e., channels) to be transmitted on a single fiber and further increases the bandwidth of an optical transmission network. Conventional WDM networks use erbium-doped fiber amplifiers (EDFAs) for providing amplification throughout a WDM network. For example, signal amplification may be necessary for maintaining an adequate signal-to-noise ratio (SNR) for optical transmission over long distances and between multiple network nodes. EDFAs provide signal amplification without requiring the signal to be converted to an electrical signal, amplified, and then converted back to an optical signal.
FIG. 1 illustrates the gain compression of a conventional EDFA. An EDFA in an optical network is conventionally operated in saturation regime A, where the largest output power is available. Moreover, in this saturation regime A, the output power can be maintained despite fluctuations in input power. Typical EDFA's used in telecommunications begin to saturate at an output power of one-to-several milliwatts. EDFA's typically reach output powers of 50-200 milliwatts when operated in a highly saturated condition. These ranges are typical and are not to be appreciated as limiting as EDFA's may be designed to still greater output power level ranges. The present large range between the output power at the onset of saturation and the largest possible output power available is a consequence of the physics of optically pumping the three level Erbium-ion system of a conventional EDFA.
Although operation of EDFA's in saturation has the above-mentioned advantage of maintaining total output power at a given level, it also has several drawbacks. The gain, as a function of wavelength, changes as the degree of saturation changes. Also, the total output power is shared by the channels. If a channel is added to a transmission system employing EDFA's, the power per channel drops. Similarly, if a channel is dropped, the power in each remaining channel increases. This change in output power takes place on a time scale of about 10-100 microseconds after the channel is added or dropped. Thus, if channels are added or dropped or if one or more channels has “bursty” data (e.g. packetized data), the output power in the channel(s) will change during that time (i.e., until the amplifier reaches a new steady-state operating condition). This time-dependent problem associated with reaching a steady-state condition when using EDFA's in a WDM network is discussed further by Sun, et al., in an article entitled, “Average Inversion Level, Modeling, and Physics of Erbium-Doped Fiber Amplifiers”, IEEE Journal of Selected Topics in Quantum Electronics, August, 1997, pp. 991-1007 and by Srivastava, et al., in an article entitled “EDFA Transient Response to Channel Loss in WDM Transmission System”, IEEE Photonics Technology Letters, March, 1997, pp. 386-388.
Many schemes have been proposed to accommodate this transient response. Sun, et al., for example, disclose a technique for modeling the time-dependant gain of EDFAs, which can be used to design a WDM network.
Zhao et al., in an article entitled “Gain Clamped Erbium-Doped Fiber Amplifiers-Modeling and Experiment”, IEEE Journal of Selected Topics in Quantum Electronics, August, 1997, pp. 1008-1012 and Yu et al., in an article entitled, “Design and Modeling of Laser-Controlled Erbium-Doped Fiber Amplifiers”, IEEE Journal of Selected Topics in Quantum Electronics, August, 1997, pp. 1013-1018 disclose controlling EDFA gain by introducing lasing at a particular wavelength.
Takahashi, et al., in an article entitled “An Output Power Stabilized Erbium-Doped Fiber Amplifier with Automatic Gain Control”, IEEE Journal of Selected Topics in Quantum Electronics, August, 1997, pp. 1019-1026, disclose controlling the gain of an EDFA by using a preamplifier system for stabilizing the input power of an EDFA.
Richards, et al., in an article entitled “A Theoretical Investigation of Dynamic All-Optical Automatic Gain Control in Multichannel EDFA's and EDFA Cascades”, IEEE Journal of Selected Topics in Quantum Electronics, August, 1997, pp. 1027-1036, examine the effects of adding/dropping channels on a chain of EDFA's and examine the effectiveness of lasing to stabilize the chain.
Recently, it has been shown that semiconductor optical amplifiers (SOA's), instead of EDFA's, can be used for WDM networks. SOA's offer potential advantages over EDFA's, such as low cost and ease of integration with other devices and platforms. When SOA's are operated under saturation, the SOA's are constrained by the power sharing affects mentioned above for EDFA's when channels are added or dropped. In addition, saturated SOA's suffer from high-frequency (several gigahertz) gain-response time that can cause crosstalk and pulse distortion on a bit-period time scale. This has made it very difficult to employ saturated SOA's in WDM systems. In general, to maximize the capacity of WDM systems, output power from the amplifier as large as possible (consistent with limits set by optical fiber nonlinear effects) is desired. This desire would drive the choice of operating conditions into the saturated region of operation of the amplifier.
Sun et al., in an article entitled “Error-Free Transmission of 32×2.5 Gbit/s DWDM Channels Over 125 km Using Cascaded In-Line Semiconductor Optical Amplifiers”, Electronic Letters, vol. 35, p. 1863 (1999), disclose using a reservoir channel to minimize power fluctuations and reduce crosstalk in a WDM system. Sun et al. operate the SOA's in saturation, and the reservoir channel is used to clamp the optical gain below a predetermined level so that data on one WDM channel has a lessened crosstalk effect on the other channels.
Other methods to reduce crosstalk between modulated channels in saturated SOA's that involve polarization multiplexing (see S. Banerjee, A. K. Srivastava, B. R. Eichenbaum, C. Wolf, Y. Sun, J. W. Sulhoff and A. R. Chraplyvy, “Polarization Multiplexing Technique to Mitigate WDM Crosstalk in SOA's,” in Proc. ECOC '99, Nice, France, 9/99, paper PD3-9, pp. 62-63 and J. Yu, X. Zheng and P. Jeppesen, OSA Topical Meeting on Optical Amplifiers and their Applications, Quebec, Canada, 7/00, paper 0TuB2) or frequency-division multiplexing of complementary channels have been proposed (see Hyang K. Kim and S. Chandrasekhar, “Reduction of Cross-Gain Modulation in the SOA by Using Wavelength Modulated Signal,” IEEE Photon. Technol. Lett. 12.10, 10/00, pp. 1412-1414).
L. Spiekman et al., (the present inventors) in one of other articles cited in this paragraph and not to be considered as prior art to the present application entitled “DWDM transmission of thirty-two 10 Gbits/s channels through 160 km Link Using Semiconductor Optical Amplifiers,” Electron. Lett. 36.12, 6/00, pp. 1046-1047 disclose WDM transmission using SOA's either operated with a reservoir channel or operated very lightly in saturation to reduce crosstalk. In other recent work, the present inventors have demonstrated that operation of SOA's in the very light saturation regime is effective for high-capacity WDM systems operating at 10 or 20 Gbits/s”; (see Spiekman et al., “8×10 Gb/s DWDM Transmission over 240 km of Standard Fiber Using a Cascade of SOA's,” IEEE Photon. Technol. Lett. 12.8, 8/00, pp. 1082-1084 and “Transmission of 8 DWDM Channels at 20 Gb/s over 160 km of Standard Fiber Using a Cascade of Semiconductor Optical Amplifiers,” IEEE Photon. Technol. Lett., 12, 6, 6/00, pp. 717-719. These articles by Spiekman et al. should be deemed to be incorporated by reference as to their entire contents for an understanding of the present invention.
In WDM networks, it would be beneficial to add or drop channels, for example, at network nodes, thus increasing flexibility over conventional point-to-point transmission systems including intermediate nodes where no adding or dropping would be typically permitted due to the adverse transient effects. As described above, EDFA's operated in their normal, saturation regime will be unsuitable for this application. None of the prior art schemes described above address the issue of the viability of using SOA's operating in a linear mode in a dynamic add/drop WDM network. Consequently, a need exists for controlling transient power response in a dynamic add/drop network.